The area of high power density DC/DC converters has been an important research topic, especially for switched mode power supply applications rated at up to 500 watts. The needs of the next generation of aerospace applications require extremely high power densities at power levels in the multi-kilowatt to megawatt range. The implications of realizing high power density and low weight systems at these power levels have rarely been addressed.
Recognizing that higher switching frequencies are the key to reducing the size of the transformer and filter elements, it is apparent that some form of soft switching converter with zero switching loss is required, if system efficiencies and heat sink size are to be maintained at a reasonable level. By far the most attractive circuit so far has been the series resonant converter (SRC). See F. C. Schwarz, J. B. Klaassens, "A Controllable 45-kw Current Source for DC Machines", IEEE Transactions IA, Vol. IA-15, No.4, July/August 1979, pp. 437-444. Using thyristors with a single LC circuit for device commutation and energy transfer, the topology is extremely simple in realization and offers the possiblity of power densities in the 0.9-1.0 Kg/KW range at power levels up to 100 KW.
The following problems can be identified with the SRC. Thyristor commutation requirements demand higher current ratings from the devices and higher VA ratings from the LC components. Thyristor recovery times significantly slow down the maximum switching frequencies attainable. Snubber inductors and RC networks are needed to effect current transfer without encountering a diode recovery problem. Capacitive input and output filters have to handle ripple currents at least as large as the load current. Although switching frequencies in the 10 KHz range yield dramatic reduction in converter size when compared to conventional hard switching circuits, it is clear that systems operating at similar frequencies and with lower component ratings are potentially capable of even higher power densities.
Soft switched converters are characterized by intrinsic modes of operation which allow an automatic and lossless resetting of the snubber elements through an appropriate recirculation of trapped energy. The capability to eliminate losses associated with the snubber permit the use of oversized snubbers resulting in dramatically lower device switching losses, even at substantially higher frequencies. Examples of soft switched DC/DC converters are the parallel output SRC operated above resonance, R. L. Steigerwald, "High-Frequency Resonant Transistor DC/DC Converters", IEEE Transactions on Industrial Electronics, Vol. IE-31, No. 2, May 1984, pp. 181-191, the resonant pole, the pseudo-resonant converter, D. M. Divan, O. Patterson, "A Pseudo Resonant Full Bridge DC/DC Converter", IEEE-PESC 1987, Conf. Rec. pp. 424-430., A. S. Kislovski, "Half Bridge Power Processing Cell Utilizing a Linear Variable Inductor and Thyristor Dual Switches", IEEE-PESC 1988 Conf Rec , pp. 284-289, and all quasi-resonant converters, K. H. Liu & F. C. Lee, "Zero-Voltage Switching Technique in DC/DC Converters", IEEE-PESC Conf. Rec. pp. 58-70, June 1986, W. A. Tabisz and F. C. Lee, "Zero Voltage Switching Multi-Resonant Technique - A Novel Approach to Improve Performance of High Frequency Quasi-Resonant Converters," IEEE-PESC 1988 Conf. Rec., pp. 9-17, Vinciarelli, U.S. Pat. No. 4,415,959. For multi-quadrant operation and for DC/AC inverter applications, typical examples of soft-switched topologies are the resonant DC link inverter and the quasi-resonant current mode or resonant pole inverter as discussed in the Tabisz and Lee article, supra.
The use of a MOSFET as a synchronous rectifier in low voltage, low power applications has been proposed. In this mode, MOSFET operation is synchronized with its anti-parallel diode to obtain a low forward voltage drop. See B. J. Baliga, "Modern Power Devices, " John Wiley, 1987; Fisher, Korman and Franz, "Performance of Low Loss Synchronous Rectifiers in a Series Parallel Resonant DC-DC Converter," APEC 89 Conf. Record.
The preferred DC/DC converter topology for high power applications has been the full bridge circuit operated at constant frequency under a pulse width control strategy. The topology features minimal voltage and current stresses in the devices, minimum VA rating of the high frequency transformer, as well as low ripple current levels in the output filter capacitor. The power density levels that can be reached are limited by peak and average device switching losses, transformer leakage inductances and output rectifier reverse recovery. The factors above constrain the maximum frequency attainable, and thus the smallest size possible, given the state of the art in component technology. Most of the soft switching converters reported in the literature attempt to tackle one or more of the problems listed above, typically at the expense of substantially higher component stresses. For high power operation, that is unacceptable. Soft switching variations of the full bridge converter are thus the most favoured topologies.
A pseudo-resonant DC/DC converter is described in the Divan and Patterson article, supra. It uses capacitive snubbers and can be designed with device stresses approaching that of the conventional full bridge. However, the circuit uses the transformer as a voltage transfer element and the interactions of its leakage inductance (L.sub.1) and the output rectifier are unresolved. The maximum switching frequency limit is reached when the energy lost due to L.sub.1 and the peak diode reverse recovery current become unacceptable.
It has been proposed in D. M. Divan, "Diodes as Pseudo-Active Elements in High Frequency DC/DC Converters," IEEE Trans. Power Electronics, Vol. 4, No. 1, Jan. 1989, that the diode recovery process in such circuits is akin to the existence of an active device in anti-parallel with it.